The Hindenburg disaster occurred on May 6, 1937, as the German passenger airship LZ 129 Hindenburg caught fire and was destroyed during its attempt to dock with its mooring mast at Naval Air Station Lakehurst in Manchester Township, New Jersey, United States. Of the 97 people on board (36 passengers and 61 crewmen), there were 35 fatalities (13 passengers and 22 crewmen). One worker on the ground was also killed, raising the final death toll to 36.

The disaster was the subject of spectacular newsreel coverage, photographs, and Herbert Morrison's recorded radio eyewitness reports from the landing field, which were broadcast the next day.[1] A variety of hypotheses have been put forward for both the cause of ignition and the initial fuel for the ensuing fire. The incident shattered public confidence in the giant, passenger-carrying rigid airship and marked the abrupt end of the airship era.[2]

After opening its 1937 season by completing a single round-trip passage to Rio de Janeiro, Brazil, in late March 1937, the Hindenburg departed from Frankfurt, Germany, on the evening of May 3, 1937, on the first of 10 round trips between Europe and the United States that were scheduled for its second year of commercial service. The American Airlines company had contracted with the operators of the Hindenburg to shuttle the passengers from Lakehurst to Newark for connections to airplane flights.[3]

Except for strong headwinds that slowed its progress, the Atlantic crossing of the Hindenburg was otherwise unremarkable until the airship attempted an early-evening landing at Lakehurst three days later on May 6. Although carrying only half its full capacity of passengers (36 of 70) and crewmen (61, including 21 crewman trainees) for the accident flight, the Hindenburg was fully booked for its return flight. Many of the passengers with tickets to Germany were planning to attend the coronation of King George VI and Queen Elizabeth in London the following week.

The Hindenburg over Manhattan, New York on May 6, 1937.

The airship was hours behind schedule when she passed over Boston on the morning of May 6, and her landing at Lakehurst was expected to be further delayed because of afternoon thunderstorms. Advised of the poor weather conditions at Lakehurst, Captain Max Pruss charted a course over Manhattan Island, causing a public spectacle as people rushed out into the street to catch sight of the airship. After passing over the field at 4:00 p.m., Captain Pruss took passengers on a tour over the seasides of New Jersey while waiting for the weather to clear. After finally being notified at 6:22 p.m. that the storms had passed, Pruss directed the airship back to Lakehurst to make its landing almost half a day late. However, as this would leave much less time than anticipated to service and prepare the airship for its scheduled departure back to Europe, the public was informed that they would not be permitted at the mooring location or be able to visit aboard the Hindenburg during its stay in port.

Around 7:00 p.m. local time, at an altitude of 650 feet (200 m), the Hindenburg made its final approach to the Lakehurst Naval Air Station. This was to be a high landing, known as a flying moor, because the airship would drop its landing ropes and mooring cable at a high altitude, and then be winched down to the mooring mast. This type of landing maneuver would reduce the number of ground crewmen, but would require more time. Although the high landing was a common procedure for American airships, the Hindenburg had only performed this maneuver a few times in 1936 while landing in Lakehurst.

At 7:09, the airship made a sharp full-speed left turn to the west around the landing field because the ground crew was not ready. At 7:11, it turned back toward the landing field and valved gas. All engines idled ahead and the airship began to slow. Captain Pruss ordered aft engines full astern at 7:14 while at an altitude of 394 ft (120 m), to try to brake the airship.

At 7:17, the wind shifted direction from east to southwest, and Captain Pruss ordered a second sharp turn starboard, making an s-shaped flightpath towards the mooring mast. At 7:18, as the final turn progressed, Pruss ordered 300, 300 and 500 kg of water ballast in successive drops because the airship was stern-heavy. The forward gas cells were also valved. As these measures failed to bring the ship in trim, six men (three of whom were killed in the accident)[Note 1] were then sent to the bow to trim the airship.

At 7:21, while the Hindenburg was at an altitude of 295 feet (90 m), the mooring lines were dropped from the bow; the starboard line was dropped first, followed by the port line. The port line was overtightened as it was connected to the post of the ground winch. The starboard line had still not been connected. A light rain began to fall as the ground crew grabbed the mooring lines.

At 7:25 p.m., a few witnesses saw the fabric ahead of the upper fin flutter as if gas were leaking.[4] Others reported seeing a dim blue flame – possibly static electricity, or St Elmo's Fire – moments before the fire on top and in the back of the ship near the point where the flames first appeared.[5] Several other eyewitness testimonies suggest that the first flame appeared on the port side just ahead of the port fin, and was followed by flames which burned on top. Commander Rosendahl testified to the flames in front of the upper fin being "mushroom-shaped". One witness on the starboard side reported a fire beginning lower and behind the rudder on that side. On board, people heard a muffled detonation and those in the front of the ship felt a shock as the port trail rope overtightened; the officers in the control car initially thought the shock was caused by a broken rope.

At 7:25 p.m. local time, the Hindenburg caught fire and quickly became engulfed in flames. Eyewitness statements disagree as to where the fire initially broke out; several witnesses on the port side saw yellow-red flames first jump forward of the top fin near the ventilation shaft of cells 4 and 5.[4] Other witnesses on the port side noted the fire actually began just ahead of the horizontal port fin, only then followed by flames in front of the upper fin. One, with views of the starboard side, saw flames beginning lower and farther aft, near cell 1 behind the rudders. Inside the airship, helmsman Helmut Lau, who was stationed in the lower fin, testified hearing a muffled detonation and looked up to see a bright reflection on the front bulkhead of gas cell 4, which "suddenly disappeared by the heat". As other gas cells started to catch fire, the fire spread more to the starboard side and the ship dropped rapidly. Although there were cameramen from four newsreel teams and at least one spectator known to be filming the landing, as well as numerous photographers at the scene, no known footage or photograph exists of the moment the fire started.

Wherever they started, the flames quickly spread forward first consuming cells 1 to 9, and the rear end of the structure imploded. Almost instantly, two tanks (it is disputed whether they contained water or fuel) burst out of the hull as a result of the shock of the blast. Buoyancy was lost on the stern of the ship, and the bow lurched upwards while the ship's back broke; the falling stern stayed in trim.

A fire-damaged 9" duralumin cross brace from the frame of the Hindenburg salvaged in May 1937 from the crash site at NAS Lakehurst, NJ.

As the tail of the Hindenburg crashed into the ground, a burst of flame came out of the nose, killing nine of the 12 crew members in the bow. There was still gas in the bow section of the ship, so it continued to point upward as the stern collapsed down. The cell behind the passenger decks ignited as the side collapsed inward, and the scarlet lettering reading "Hindenburg" was erased by flames as the bow descended. The airship's gondola wheel touched the ground, causing the bow to bounce up slightly as one final gas cell burned away. At this point, most of the fabric on the hull had also burned away and the bow finally crashed to the ground. Although the hydrogen had finished burning, the Hindenburg's diesel fuel burned for several more hours.

The fire bursts out of the nose of the Hindenburg.

The time that it took from the first signs of disaster to the bow crashing to the ground is often reported as 32, 34 or 37 seconds. Since none of the newsreel cameras were filming the airship when the fire started, the time of the start can only be estimated from various eyewitness accounts and the duration of the longest footage of the crash. One careful analysis by NASA's Addison Bain gives the flame front spread rate across the fabric skin as about 49 ft/s (15 m/s) at some points during the crash, which would have resulted in a total destruction time of about 16 seconds (245m/15 m/s=16.3 s).

In the days after the disaster, an official board of inquiry was set up at Lakehurst to investigate the cause of the fire. The investigation by the US Commerce was headed by Colonel South Trimble Jr, while Dr. Hugo Eckener led the German commission.

Hindenburg disaster sequence from the Pathé Newsreel, showing the bow nearing the ground.

The disaster was well-documented because of the combination of many news crews having been on site at the time of the airship exploding and the significant extent of newsreel coverage and photographs, as well as Herbert Morrison's eyewitness radio report for station WLS in Chicago, which was broadcast the next day. Heavy publicity about the first transatlantic passenger flight of the year by Zeppelin to the United States attracted a large number of journalists to the landing. (The airship had already made one round trip from Germany to Brazil that year.)

Morrison's broadcast remains one of the most famous in history. Parts of it were later dubbed onto newsreel footage, giving the impression that the words and film were recorded together. His plaintive, "Oh, the humanity!" has been widely used in popular culture. Part of the poignancy of his commentary is the result of its having been recorded at a slightly slower speed, so that when it is played back at normal speed, it seems to have a faster delivery and higher pitch. When corrected for pitch without adjusting the speed, Morrison's account sounds less frantic.[citation needed]

It's practically standing still now they've dropped ropes out of the nose of the ship; and (uh) they've been taken ahold of down on the field by a number of men. It's starting to rain again; it's... the rain had (uh) slacked up a little bit. The back motors of the ship are just holding it (uh) just enough to keep it from...It's burst into flames! Get this, Charlie; get this, Charlie! It's fire... and it's crashing! It's crashing terrible! Oh, my! Get out of the way, please! It's burning and bursting into flames and the... and it's falling on the mooring mast. And all the folks agree that this is terrible; this is the worst of the worst catastrophes in the world. Oh it's... [unintelligible] its flames... Crashing, oh! Four- or five-hundred feet into the sky and it... it's a terrific crash, ladies and gentlemen. It's smoke, and it's in flames now; and the frame is crashing to the ground, not quite to the mooring mast. Oh, the humanity! And all the passengers screaming around here. I told you; it – I can't even talk to people, their friends are on there! Ah! It's... it... it's a... ah! I... I can't talk, ladies and gentlemen. Honest: it's just laying there, mass of smoking wreckage. Ah! And everybody can hardly breathe and talk and the screaming. I... I... I'm sorry. Honest: I... I can hardly breathe. I... I'm going to step inside, where I cannot see it. Charlie, that's terrible. Ah, ah... I can't. Listen, folks; I... I'm gonna have to stop for a minute because I've lost my voice. This is the worst thing I've ever witnessed.

The iconic newsreel footage was shot by Al Gold of Fox Movietone News who later received a Presidential Citation for his work.[7][8] His film along with Morrison's passionate reporting shattered public and industry faith in airships and marked the end of the giant passenger-carrying airships. Also contributing to the downfall of Zeppelins was the arrival of international passenger air travel and Pan American Airlines. Heavier-than-air aircraft regularly crossed the Atlantic and Pacific much faster than the 130 km/h (80 mph) speed of the Hindenburg. The one advantage that the Hindenburg had over such aircraft was the comfort that she afforded her passengers, much like that of an ocean liner.

There had been a series of other airship accidents prior to the Hindenburg fire; many were caused by bad weather. The Graf Zeppelin had flown safely for more than 1.6 million kilometers (1.0 million miles), including the first circumnavigation of the globe by an airship. The Zeppelin company's promotions had prominently featured the fact that no passenger had been injured on any of its airships.

Despite the sheer ferocity of the colossal fire, many of the crew members and passengers survived, but the majority of them were severely burned. Of the 36 passengers and 61 crewmen, 13 passengers and 22 aircrewmen died. Also killed was one ground crewman, the civilian linesman Allen Hagaman.[9] Ten passengers[10] and 16 crewmen[11] died in the crash or in the fire. The majority of the victims were burnt to death, while others died jumping from the airship at an excessive height, or as a consequence of either smoke inhalation or falling debris.[12] while six other crewmembers,[13] three passengers[14] and Allen Hagaman died in the following hours or days, mostly as a consequence of the burns.[15]

The majority of the crewmen who died were up inside the ship's hull, where they either did not have a clear escape route or else were close to the bow of the ship, which hung burning in the air too long for most of them to escape the fire. Most of the passengers who died were trapped in the starboard side of the passenger deck. Not only was the wind blowing the fire toward the starboard side, but the ship also rolled slightly to starboard as it settled to the ground, with much of the upper hull on that part of the ship collapsing outboard of the starboard observation windows, thus cutting off the escape of many of the passengers on that side.[Note 2] To make matters worse, the sliding door leading from the starboard passenger area to the central foyer and the gangway stairs (through which rescuers led a number of passengers to safety) jammed shut during the crash, further trapping those passengers on the starboard side.[Note 3] Nonetheless, some did manage to escape from the starboard passenger decks. By contrast, all but a few of the passengers on the port side of the ship survived the fire, with some of them escaping virtually unscathed. Although the most famous airship disaster, it was not the worst. Just over twice as many perished (73 of 76 on board) when the helium-filled U.S. Navy scout airship USS Akron crashed at sea off the New Jersey coast four years earlier on April 4, 1933.[16]

Some of the survivors were saved by luck. Werner Franz, the 14-year-old cabin boy, was initially dazed by the realization that the ship was on fire. As he stood near the officer's mess where he had been putting away dishes moments before, a water tank above him burst open, and he was suddenly soaked to the skin. Not only did this snap him back to his senses, as he later told interviewers, but it also put out the fire around him. He then made his way to a nearby hatch through which the kitchen had been provisioned before the flight, and dropped through it just as the forward part of the ship was briefly rebounding into the air. He began to run toward the starboard side, but stopped and turned around and ran the other way, because the flames were being pushed by the wind in that same direction. He made it clear of the wreck with no injuries, and lived to be the last surviving crew member when he died at age 92 on August 13, 2014.[17] The last surviving passenger is Werner G. Doehner (b. 1928), a retired electrical engineer who was an eight-year-old child traveling with his parents, brother, and sister at the time of the accident. His father and sister died in the disaster.[18]

When the control car crashed on the ground, most of the officers had leapt through the windows, but became separated. First Officer Captain Albert Sammt found Captain Max Pruss trying to re-enter the wreckage to look for survivors. Pruss's face was badly burned, and he required months of hospitalization and reconstructive surgery, but he survived.[19]

Captain Ernst Lehmann escaped the crash with burns to his head and arms and severe burns across most of his back. He died at a nearby hospital the next day.[20]

When passenger Joseph Späh, a vaudeville comic acrobat, saw the first sign of trouble he smashed the window with his movie camera, with which he had been filming the landing (the film survived the disaster). As the ship neared the ground he lowered himself out the window and hung onto the window ledge, letting go when the ship was perhaps 20 feet above the ground. His acrobat's instincts kicked in, and Späh kept his feet under him and attempted to do a safety roll when he landed. He injured his ankle nonetheless, and was dazedly crawling away when a member of the ground crew came up, slung the diminutive Späh under one arm, and ran him clear of the fire.[Note 4][Note 5]

Of the 12 crewmen in the bow of the airship, only three survived. Four of these 12 men were standing on the mooring shelf, a platform up at the very tip of the bow from which the forwardmost landing ropes and the steel mooring cable were released to the ground crew, and which was directly at the forward end of the axial walkway and just ahead of gas cell #16. The rest were standing either along the lower keel walkway ahead of the control car, or else on platforms beside the stairway leading up the curve of the bow to the mooring shelf. During the fire the bow hung in the air at roughly a 45-degree angle and flames shot forward through the axial walkway, bursting through the bow (and the bow gas cells) like a blowtorch. The three men from the forward section who survived (elevatorman Kurt Bauer, cook Alfred Grözinger, and electrician Josef Leibrecht) were those furthest aft of the bow, and two of them (Bauer and Grözinger) happened to be standing near two large triangular air vents, through which cool air was being drawn by the fire. Neither of these men sustained more than superficial burns.[Note 6] Most of the men standing along the bow stairway either fell aft into the fire, or tried to leap from the ship when it was still too high in the air. Three of the four men standing on the mooring shelf inside the very tip of the bow were actually taken from the wreck alive, though one (Erich Spehl, a rigger) died shortly afterward in the Air Station's infirmary, and the other two (helmsman Alfred Bernhard and apprentice elevatorman Ludwig Felber) were reported by newspapers to have initially survived the fire, and then to subsequently have died at area hospitals during the night or early the following morning.[citation needed]

The four crewmen in the tail fin all survived. They were closest to the origin of the fire but sheltered by the structure of the lower fin. They escaped by climbing out the fin's access hatch when the tail hit the ground.[citation needed]

Hydrogen fires are notable for being less destructive to immediate surroundings than gasoline explosions because of the buoyancy of H2, which causes heat of combustion to be released upwards more than circumferentially as the leaked mass ascends in the atmosphere; hydrogen fires are more survivable than fires of gasoline and of wood.[22] The hydrogen in the Hindenburg burned out within about 90 seconds.

At the time of the disaster, sabotage was commonly put forward as the cause of the fire, initially by Hugo Eckener, former head of the Zeppelin Company and the "old man" of German airships. In initial reports, before inspecting the accident, Eckener mentioned the possibility of a shot as the cause of the disaster, because of threatening letters that had been received, but did not rule out other causes.[23] Eckener later publicly endorsed the static spark hypothesis. At the time on a lecture tour in Austria, he was awakened at about 2:30 in the morning (8:30 p.m. Lakehurst time, or approximately an hour after the crash) by the ringing of his bedside telephone. It was a Berlin representative of the New York Times with news that the Hindenburg "exploded yesterday evening at 7 p.m. [sic] above the airfield at Lakehurst." By the time he left the hotel the next morning to travel to Berlin for a briefing on the disaster, the only answer that he had for the reporters waiting outside to question him was that based on what he knew, the Hindenburg had "exploded over the airfield"; sabotage might be a possibility. However, as he learned more about the disaster, particularly that the airship had burned rather than actually "exploded", he grew more and more convinced that static discharge, rather than sabotage, was the cause.[24][page needed]

Commander Charles Rosendahl, commander of the Naval Air Station at Lakehurst and the man in overall charge of the ground-based portion of the Hindenburg's landing maneuver, also came to believe that the Hindenburg had been sabotaged. He laid out a general case for sabotage in his 1938 book What About the Airship?,[25][page needed] which was as much an extended argument for the further development of the rigid airship as it was a historical overview of the airship.

Another proponent of the sabotage hypothesis was Max Pruss, commander of the Hindenburg throughout the airship's career. Pruss flew on nearly every flight of the Graf Zeppelin until the Hindenburg was ready. In a 1960 interview conducted by Kenneth Leish for Columbia University's Oral History Research Office, Pruss said early dirigible travel was safe, and therefore he strongly believed that sabotage was to blame. He stated that on trips to South America, which was a popular destination for German tourists, both airships passed through thunderstorms and were struck by lightning but remained unharmed.[26]

Most members of the crew refused to believe that one of them would commit an act of sabotage, insisting only a passenger could have destroyed the airship. A suspect favored by Commander Rosendahl, Captain Pruss, and others among the Hindenburg's crew, was passenger Joseph Späh, a German acrobat who survived the fire. He brought with him a dog, a German shepherd named Ulla, as a surprise for his children (Ulla did not survive the crash). He reportedly made a number of unaccompanied visits to feed his dog, who was being kept in a freight room near the stern of the ship. Those who suspected Späh based their suspicions primarily on those trips into the ship's interior to feed his dog, that according to some of the stewards Späh had told anti-Nazi jokes during the flight, recollections by stewards that Späh had seemed agitated by the repeated delays in landing, and that he was an acrobat who could conceivably climb into the airship's rigging to plant a bomb.

In 1962, A. A. Hoehling published Who Destroyed the Hindenburg?, in which he rejected all theories but sabotage, and named a crew member as the suspect. Eric Spehl, a rigger on the Hindenburg who died in the fire, was named as the saboteur. Ten years later, Michael MacDonald Mooney's book The Hindenburg, which was based heavily on Hoehling's sabotage hypothesis, also identified Spehl as the saboteur; Mooney's book was made into the movie The Hindenburg, the producers of which were sued by Hoehling for plagiarism, but Hoehling lost because he had presented his sabotage hypothesis as historical fact, and one cannot claim ownership of historical facts.[27]

Hoehling claimed the following in naming Spehl as the culprit:

Spehl's girlfriend had communist beliefs and anti-Nazi connections.

The fire's origin was near the catwalk running through Gas Cell 4, which was an area of the ship generally off-limits to anyone other than Spehl and his fellow riggers.

Rumors that the Gestapo had investigated Spehl's possible involvement in 1938.

Spehl's interest in amateur photography, making him familiar with flashbulbs that could have served as an igniter.

The discovery by representatives of the NYPD Bomb Squad of a substance that was later determined to likely be "the insoluble residue from the depolarizing element of a small, dry battery." (Hoehling postulated that a dry cell battery could have powered a flashbulb in an incendiary device.)

The discovery by FBI Agents of a yellow substance on the valve cap of the airship between cells 4 and 5 where the fire was first reported. Some have suggested this to be sulfur, which can ignite hydrogen. (However, a further investigation into this suggested that the residue was actually from a fire extinguisher in the stern of the ship.)

A flash or a bright reflection in gas cell 4, that crew members near the lower fin had seen just before the fire.

Hoehling's (and later Mooney's) hypothesis goes on to say that it is unlikely that Spehl wanted to kill people, and that he intended the airship to burn after the landing. However, with the ship already over 12 hours late, Spehl was unable to find an excuse to reset the timer on his bomb.

It has been suggested that Adolf Hitler himself ordered the Hindenburg to be destroyed in retaliation for Eckener's anti-Nazi opinions.[28]

Since the publication of Hoehling's book, most airship historians, including Dr. Douglas Robinson, have dismissed Hoehling's sabotage hypothesis because no solid evidence was ever presented to support it. No pieces of a bomb were ever discovered (and there is no evidence in existing documentation that the sample collected from the wreckage, and determined to be residue from a dry cell battery, was found anywhere near the stern of the airship), and on closer examination, the evidence against Spehl and his girlfriend turned out to be rather weak. Hoehling himself asserted it was only his theory and also suggested a short-circuit could be another potential cause of the fire. Additionally, Mooney's book has been disregarded to contain numerous elements which are fictional, perhaps as a plot for the then-upcoming 1975 film.[29] Although Mooney alleges that three Luftwaffe officers were aboard to investigate a potential bomb threat, there is no evidence they were on board to do so, and military observers were present on previous flights to study navigational techniques and weather forecasting practices of the airship crew.[30]

However, opponents of the sabotage hypothesis argued that only speculation supported sabotage as a cause of the fire, and no credible evidence of sabotage was produced at any of the formal hearings. Eric Spehl died in the fire and was therefore unable to refute the accusations that surfaced a quarter of a century later. The FBI investigated Joseph Späh and reported finding no evidence of Späh having any connection to a sabotage plot. According to his wife, Evelyn, Späh was quite upset over the accusations – she later recalled that her husband was outside their home cleaning windows when he first learned that he was suspected of sabotaging the Hindenburg, and was so shocked by the news that he almost fell off the ladder on which he was standing.[31]

Neither the German nor the American investigation endorsed any of the sabotage theories. Proponents of the sabotage hypothesis argue that any finding of sabotage would have been an embarrassment for the Nazi regime, and they speculate that such a finding by the German investigation was suppressed for political reasons. However, it has also been suggested that numerous crewmen subscribed to the sabotage hypothesis because they refused to accept any flaws with the airship or pilot error.[32]

Some more sensational newspapers claimed that a Luger pistol with one round fired was found among the wreckage and speculated that a person on board committed suicide or shot the airship.[33][34] There is no other evidence suggesting an attempted suicide or official report confirming the presence of a Luger pistol.[citation needed] Initially, before inspecting the scene himself, Eckener mentioned the possibility for a shot as the cause of the disaster, because of threatening letters they received.[23] At the German enquiry Eckener discounted a shot – among many possibilities – as the cause as nearly impossible and highly improbable.[35]

Hugo Eckener argued that the fire was started by an electric spark which was caused by a buildup of static electricity on the airship.[36] The spark ignited hydrogen on the outer skin.

Proponents of the static spark hypothesis point out that the airship's skin was not constructed in a way that allowed its charge to be distributed evenly throughout the craft. The skin was separated from the duralumin frame by non-conductive ramie cords which had been lightly covered in metal to improve conductivity but not very effectively, allowing a large difference in potential to form between the skin and the frame.

In order to make up for the delay of more than 12 hours in its transatlantic flight, the Hindenburg passed through a weather front of high humidity and high electrical charge. Although the mooring lines were not wet when they first hit the ground and ignition took place four minutes after, Eckener theorised that they may have become wet in these four minutes. When the ropes, which were connected to the frame, became wet, they would have grounded the frame but not the skin. This would have caused a sudden potential difference between skin and frame (and the airship itself with the overlying air masses) and would have set off an electrical discharge – a spark. Seeking the quickest way to ground, the spark would have jumped from the skin onto the metal framework, igniting the leaking hydrogen.

In his 1964 book LZ-129 Hindenburg, Zeppelin historian Dr. Douglas Robinson points out that although ignition of free hydrogen by static discharge had become a favored hypothesis, no such discharge was seen by any of the witnesses who testified at the official investigation into the accident back in 1937. He goes on to write:

But within the past year, I have located an observer, Professor Mark Heald of Princeton, New Jersey, who undoubtedly saw St. Elmo's Fire flickering along the airship's back a good minute before the fire broke out. Standing outside the main gate to the Naval Air Station, he watched, together with his wife and son, as the Zeppelin approached the mast and dropped her bow lines. A minute thereafter, by Mr. Heald's estimation, he first noticed a dim "blue flame" flickering along the backbone girder about one-quarter the length abaft the bow to the tail. There was time for him to remark to his wife, "Oh, heavens, the thing is afire," for her to reply, "Where?" and for him to answer, "Up along the top ridge" – before there was a big burst of flaming hydrogen from a point he estimated to be about one-third the ship's length from the stern.[37]

Unlike other witnesses to the fire whose view of the port side of the ship had the light of the setting sun behind the ship, Professor Heald's view of the starboard side of the ship against a backdrop of the darkening eastern sky would have made the dim blue light of a static discharge on the top of the ship more easily visible.

Harold G. Dick was Goodyear Zeppelin's representative with Luftschiffbau Zeppelin during the mid-1930s. He flew on test flights of the Hindenburg and its sister ship, the Graf Zeppelin II. He also flew on numerous flights in the original Graf Zeppelin and ten round-trip crossings of the north and south Atlantic in the Hindenburg. In his book The Golden Age of the Great Passenger Airships Graf Zeppelin & Hindenburg, he observes:

There are two items not in common knowledge. When the outer cover of the LZ 130 [the Graf Zeppelin II] was to be applied, the lacing cord was prestretched and run through dope as before but the dope for the LZ 130 contained graphite to make it conductive. This would hardly have been necessary if the static discharge hypothesis were mere cover-up. The use of graphite dope was not publicized and I doubt if its use was widely known at the Luftschiffbau Zeppelin.

In addition to Dick's observations is the fact that during the Graf Zeppelin II's early test flights, measurements were taken of the airship's static charge. It is clear that Dr. Ludwig Durr and the other engineers at Luftschiffbau Zeppelin took the static discharge hypothesis seriously and considered the insulation of the fabric from the frame to be a design flaw in the Hindenburg. Thus, the German Inquiry concluded that the insulation of the outer covering caused a spark to jump onto a nearby piece of metal, thereby igniting the hydrogen. In lab experiments, using the Hindenburg's outer covering and a static ignition, hydrogen was able to be ignited but with the covering of the LZ 127 Graf Zeppelin, nothing happened. These findings were not well-publicized and covered up, perhaps to avoid embarrassment of such an engineering flaw in the face of the Third Reich.

A variant of the static spark hypothesis, presented by Addison Bain, is that a spark between inadequately grounded fabric cover segments of the Hindenburg itself started the fire, and that the spark had ignited the "highly flammable" outer skin. The Hindenburg had a cotton skin covered with a finish known as "dope". It is a common term for a plasticisedlacquer that provides stiffness, protection, and a lightweight, airtight seal to woven fabrics. In its liquid forms, dope is highly flammable, but the flammability of dry dope depends upon its base constituents, with, for example, butyrate dope being far less flammable than cellulose nitrate. Proponents of this hypothesis claim that when the mooring line touched the ground, a resulting spark could have ignited the dope in the skin.

An episode of the Discovery Channel series Curiosity entitled "What Destroyed the Hindenburg?", which first aired in December 2012, investigated both the static spark theory and St. Elmo's Fire, as well as sabotage by bomb. The team, led by British aeronautical engineer Jem Stansfield and American airship historian Dan Grossman, concluded that the ignition took place above the hydrogen vent just forward of where Mark Heald saw St. Elmo's Fire, and that the ignited hydrogen was channelled down the vent where it created a more explosive detonation described by crew member Helmut Lau. "What Destroyed the Hindenburg?" was aired as a standalone documentary on the UK's Channel 4 in March 2013.[38]

A. J. Dessler, former director of the Space Science Laboratory at NASA's Marshall Space Flight Center and a critic of the incendiary paint hypothesis (see below), favors a much simpler explanation for the conflagration: lightning. Like many other aircraft, the Hindenburg had been struck by lightning several times. This does not normally ignite a fire in hydrogen-filled airships, because the hydrogen is not mixed with oxygen. However, many fires started when lightning struck airships as they were venting hydrogen as ballast in preparation for landing, which the Hindenburg was doing at the time of the disaster. Hindenburg was a huge airship so there was a big chance of it being struck. The vented hydrogen mixes with the air, making it a combustible mixture.

However, witnesses did not observe any lightning storms as the ship made its final approach.

On the 70th anniversary of the accident, The Philadelphia Inquirer carried an article[39] with yet another hypothesis, based on an interview of ground crew member Robert Buchanan. He had been a young man on the crew manning the mooring lines.

As the airship was approaching the mooring mast, he noted that one of the engines, thrown into reverse for a hard turn, backfired, and a shower of sparks was emitted. After being interviewed by Addison Bain, Buchanan believed that the airship's outer skin was ignited by engine sparks. Another ground crewman, Robert Shaw, saw a blue ring behind the tail fin and had also seen sparks coming out of the engine.[40] Shaw believed that the blue ring he saw was leaking hydrogen which was ignited by the engine sparks.

Dr. Eckener rejected the idea that hydrogen could have been ignited by an engine backfire, postulating that the hydrogen could not have been ignited by any exhaust because the temperature is too low to ignite the hydrogen. The ignition temperature for hydrogen is 500 °C (932 °F), but the sparks from the exhaust only reach 250 °C (482 °F).[32] The Zeppelin Company also carried out extensive tests and hydrogen had never ignited. Additionally, the fire was first seen at the top of the airship, not near the bottom of the hull.

Most current analyses of the fire assume ignition due to some form of electricity as the cause. However, there is still much controversy over whether the fabric skin of the airship, or the hydrogen used for buoyancy, was the initial fuel for the resulting fire.

The incendiary paint theory (IPT) was proposed in 1996 by retired NASA scientist Addison Bain, stating that the doping compound of the airship was the cause of the fire, and that the Hindenburg would have burned even if it were filled with helium. The hypothesis is limited to the source of ignition and to the flame front propagation, not to the source of most of the burning material, as once the fire started and spread the hydrogen clearly must have burned (although some proponents of the incendiary paint theory claim that hydrogen burned much later in the fire or that it otherwise did not contribute to the rapid spread of the fire). The incendiary paint hypothesis asserts that the major component in starting the fire and feeding its spread was the canvas skin because of the compound used on it.

Proponents of this hypothesis point out that the coatings on the fabric contained both iron oxide and aluminum-impregnated cellulose acetate butyrate (CAB)[citation needed] which remain potentially reactive even after fully setting. Iron oxide and aluminum can be used as components of solid rocket fuel or thermite. For example, the propellant for the Space Shuttle solid rocket booster includes both "aluminum (fuel, 16%), (and) iron oxide (a catalyst, 0.4%)". The coating applied to the Hindenburg's covering did not have a sufficient quantity of any material capable of acting as an oxidizer,[41] which is a necessary component of rocket fuel,[42] however, oxygen is also available from the air.

Bain received permission from the German government to search their archives and discovered evidence that, during the Nazi regime, German scientists concluded the dope on the Hindenburg's fabric skin was the cause of the conflagration. Bain interviewed the wife of the investigation's lead scientist Max Dieckmann, and she stated that her husband had told her about the conclusion and instructed her to tell no one, presumably because it would have embarrassed the Nazi government.[43] Additionally, Dieckmann concluded that it was the poor conductivity, not the flammability of the doping compound, that led to the ignition of hydrogen.[33] However, Otto Beyersdorff, an independent investigator hired by the Zeppelin Company, asserted that the outer skin itself was flammable. In several television shows, Bain attempted to prove the flammability of the fabric by igniting it with a Jacob's Ladder. Although Bain's fabric ignited, critics point out that Bain had to correctly position the fabric parallel to a machine with a continuous electric current inconsistent with atmospheric conditions. In response to this criticism, the IPT therefore postulates that a spark would need to be parallel to the surface, and that "panel-to-panel arcing" occurs, where the spark moves between panels of paint isolated from each other. A.J. Dessler, a critic of the IPT, points out a static spark does not have sufficient energy to ignite the doping compound, and that the insulating properties of the doping compound prevents a parallel spark path through it. Additionally, Dessler contends that the skin would also be electrically conductive in the wet and damp conditions before the fire.[44]

Critics also point out that port side witnesses on the field, as well as crew members stationed in the stern, saw a glow inside Cell 4 before any fire broke out of the skin, indicating that the fire began inside the airship or that after the hydrogen ignited, the invisible fire fed on the gas cell material. Newsreel footage clearly shows that the fire was burning inside the structure.[31]

Proponents of the paint hypothesis claim that the glow is actually the fire igniting on the starboard side, as seen by some other witnesses. From two eyewitness statements, Bain asserts the fire began near cell 1 behind the tail fins and spread forward before it was seen by witnesses on the port side. However, photographs of the early stages of the fire show the gas cells of the Hindenburg's entire aft section fully aflame, and no glow is seen through the areas where the fabric is still intact. Burning gas spewing upward from the top of the airship was causing low pressure inside, allowing atmospheric pressure to press the skin inwards.

The wreckage of the Hindenburg the morning after the crash. Note the fabric remaining on the tail fins.

Occasionally the Hindenburg's varnish is incorrectly identified as, or stated being similar to, cellulose nitrate which, like most nitrates, burns very readily.[28] Instead, the cellulose acetate butyrate (CAB) used to seal the zeppelin's skin is rated by the plastics industry as combustible but nonflammable. That is, it will burn if placed within a fire but is not readily ignited. It should also be noted that not all fabric on the Hindenburg burned.[45] For example, the fabric on the port and starboard tail fins was not completely consumed. That the fabric not near the hydrogen fire did not burn is not consistent with the "explosive" dope hypothesis.

The TV show MythBusters explored the incendiary paint hypothesis. Their findings indicated that the aluminum and iron oxide ratios in the Hindenburg's skin, while certainly flammable, were not enough on their own to destroy the zeppelin. Had the skin contained enough metal to produce pure thermite, the Hindenburg would have been too heavy to fly. The MythBusters team also discovered that the Hindenburg's coated skin had a higher ignition temperature than that of untreated material, and that it would initially burn slowly, but that after some time the fire would begin to accelerate considerably. From this, they concluded that those arguing against the incendiary paint theory may have been wrong about the airship's skin being not easily set on fire due to being separated in different layers. Despite this, they still considered the skin alone to burn too slowly to account for the rapid spread of the fire. The Mythbusters concluded that the paint may have contributed to the disaster, but that it was not the sole reason for such rapid combustion.[46]

The theory that hydrogen was ignited by a static spark is the most widely accepted theory as determined by the official crash investigations. Offering support for the hypothesis that there was some sort of hydrogen leak prior to the fire is that the airship remained stern-heavy before landing, despite efforts to put the airship back in trim. This could have been caused by a leak of the gas, which started mixing with air, potentially creating a form of oxyhydrogen and filling up the space between the skin and the cells.[32] A ground crew member, R.H. Ward, reported seeing the fabric cover of the upper port side of the airship fluttering, "as if gas was rising and escaping" from the cell. He said that the fire began there, but that no other disturbance occurred at the time when the fabric fluttered.[32] Another man on the top of the mooring mast had also reported seeing a flutter in the fabric as well.[47] Pictures that show the fire burning along straight lines that coincide with the boundaries of gas cells suggest that the fire was not burning along the skin, which was continuous. Crew members stationed in the stern reported actually seeing the cells burning.[48]

Two main theories have been postulated as to how gas could have leaked. Dr. Eckener believed a snapped bracing wire had torn a gas cell open (see below), while others suggest that a maneuvering gas valve was stuck open and gas from cell 4 leaked through. During the airship's first flight to Rio, a gas cell was nearly emptied when a vent was stuck open, and gas had to be transferred from other cells to maintain an even keel.[31] Additionally, gas was constantly vented from the airship's automatic valve as pressure changed.

Although proponents of the IPT claim that the hydrogen was odorized with garlic,[49][50] it would have been detectable only in the area of a leak. Once the fire was underway, more powerful smells would have masked any garlic odor. There were no reports of anyone smelling garlic during the flight, but no official documents have been found to prove that the hydrogen was even odorized.

Opponents of this hypothesis note that the fire was reported as burning bright red, while pure hydrogen burns blue if it is visible at all,[51] although there were many other materials that were consumed by the fire which could have changed its hue.

Some of the airshipmen at the time, including Captain Pruss, asserted that the stern heaviness was normal, since aerodynamic pressure would push rainwater towards the stern of the airship. The stern heaviness was also noticed minutes before the airship made its sharp turns for its approach (ruling out the snapped wire theory), and crew members stated that it was corrected as the ship stopped (after sending six men into the bow section of the ship). Additionally, the gas cells of the ship were not pressurized, and a leak would not cause the fluttering of the outer cover, which was not seen until seconds before the fire. However, reports of the amount of rain the ship had collected have been inconsistent. Several witnesses testified that there was no rain as the ship approached until a light rain fell minutes before the fire, while several crew members stated that before the approach the ship did encounter heavy rain. Albert Sammt, the ship's first officer, initially attributed the stern-heaviness to fuel consumption and sending crewmen to their landing stations in the rear of the ship, though years later he would later assert that a leak of hydrogen had occurred. On its final approach the rainwater may have evaporated and may not completely account for the observed stern-heaviness, as the airship should have been in good trim ten minutes after passing through rain. Dr. Eckener noted that the stern heaviness was significant enough that 70,000 kilogram·meter (506,391 foot-pounds) of trimming was needed.[52]

Although Captain Pruss believed that the Hindenburg could withstand tight turns without significant damage, proponents of the puncture hypothesis, including Hugo Eckener, question the airship's structural integrity after being repeatedly stressed over its flight record.

The airship did not receive much in the way of routine inspections even though there was evidence of at least some damage on previous flights. It is not known whether that damage was properly repaired or even whether all the failures had been found. During the ship's first return flight from Rio Hindenburg had once lost an engine and almost drifted over Africa, where it could have crashed. Afterwards, Dr. Eckener ordered section chiefs to inspect the airship during flight. However, the complexity of the airship's structure would make it virtually impossible to detect all weaknesses in the structure. In March 1936, the Hindenburg and the Graf Zeppelin made three-day flights to drop leaflets and broadcast speeches via loudspeaker. Before the airship's takeoff on March 26, 1936, Ernst Lehmann chose to launch the Hindenburg with the wind blowing from behind the airship, instead of into the wind as per standard procedure. During the takeoff, the airship's tail struck the ground, and part of the lower fin was broken.[53] Although that damage was repaired, the force of the impact may have caused internal damage. Only six days before the disaster, it was planned to make the Hindenburg have a hook on her hull to carry aircraft in a similar way to what the US Navy did with the USS Akron and the USS Macon. However, the trials were unsuccessful as the biplane hit the Hindenburg's trapeze several times. The structure of the airship may have been further affected by this incident.

Newsreels as well as the log of the landing approach show the Hindenburg made several sharp turns, first towards port and then starboard, just before the accident. Proponents posit that either of these turns could have weakened the structure near the vertical fins, causing a bracing wire to snap and puncture at least one of the internal gas cells. Additionally, some of the bracing wires may have even been substandard. One bracing wire tested after the crash broke at a mere 70% of its rated load.[31] A punctured cell would have freed hydrogen into the air and could have been ignited by a static discharge (see above), or it is also possible that the broken bracing wire struck a girder causing sparks to ignite hydrogen.[31] When the fire started, people on board the airship reported hearing a muffled detonation, but outside a ground crew member on the starboard side reported hearing a crack. Some speculate the sound was from a bracing wire snapping.[31]

Eckener concluded that the puncture hypothesis, due to pilot error, was the most likely explanation for the disaster. He held Captains Pruss and Lehmann, and Charles Rosendahl responsible for what he viewed as a rushed landing procedure with the airship badly out of trim under poor weather conditions. Pruss had made the sharp turn under Lehmann's pressure; while Rosendahl called the airship in for landing, believing the conditions were suitable. Eckener noted that a smaller storm front followed the thunderstorm front, creating conditions suitable for static sparks.

During the US inquiry, Eckener testified that he believed that the fire was caused by the ignition of hydrogen by a static spark:

The ship proceeded in a sharp turn to approach for its landing. That generates extremely high tension in the after part of the ship, and especially in the center sections close to the stabilizing fins which are braced by shear wires. I can imagine that one of these shear wires parted and caused a rent in a gas cell. If we will assume this further, then what happened subsequently can be fitted in to what observers have testified to here: Gas escaped from the torn cell upwards and filled up the space between the outer cover and the cells in the rear part of the ship, and then this quantity of gas which we have assumed in the hypothesis was ignited by a static spark.

Under these conditions, naturally, the gas accumulated between the gas cells and the outer cover must have been a very rich gas. That means it was not an explosive mixture of hydrogen, but more of a pure hydrogen. The loss of gas must have been appreciable.

I would like to insert here, because the necessary trimming moments to keep the ship on an even keel were appreciable, and everything apparently happened in the last five or six minutes, that is, during the sharp turn preceding the landing maneuver, that therefore there must have been a rich gas mixture up there, or possibly pure gas, and such gas does not burn in the form of an explosion. It burns off slowly, particularly because it was in an enclosed space between outer cover and gas cells, and only in the moment when gas cells are burned by the burning off of this gas, then the gas escapes in greater volume, and then the explosions can occur, which have been reported to us at a later stage of the accident by so many witnesses.

The rest it is not necessary for me to explain, and in conclusion, I would like to state this appears to me to be a possible explanation, based on weighing all of the testimony that I have heard so far.[54]

However, the apparent stern heaviness during the landing approach was noticed thirty minutes before the landing approach, indicating that a gas leak resulting from a sharp turn did not cause the initial stern heaviness.[54]

The 2001 documentary Hindenburg Disaster: Probable Cause suggested that 16-year-old Bobby Rutan, who claimed that he had smelled "gasoline" when he was standing below the Hindenburg's aft port engine, had detected a diesel fuel leak. During the investigation, Commander Charles Rosendahl dismissed the boy's report. The day before the disaster, a fuel pump had broken during the flight, but the chief engineer testified that the pump had been replaced. The resulting vapor of a diesel leak, in addition to the engines being overheated, would have been highly flammable and could have self-combusted.

However, the documentary makes numerous mistakes into assuming that the fire began in the keel. First, it implies that the crewmen in the lower fin had seen the fire start in the keel and that Hans Freund and Helmut Lau looked towards the front of the airship to see the fire, when Freund was actually looking rearward when the fire started. Most witnesses on the ground reported seeing flames at the top of the ship, but the only location where a fuel leak could have a potential ignition source is the engines. Additionally, while investigators in the documentary suggest it is possible for a fire in the keel to go unnoticed until it breaks the top section, other investigators such as Greg Feith consider it unlikely.

Regardless of the source of ignition or the initial fuel for the fire, there remains the question of what caused the rapid spread of flames along the length of the airship. Here again the debate has centered on the fabric covering of the airship and the hydrogen used for buoyancy.

Proponents of both the incendiary paint hypothesis and the hydrogen hypothesis agree that the fabric coatings were probably responsible for the rapid spread of the fire. The combustion of hydrogen is not usually visible to the human eye in daylight, because most of its radiation is not in the visible portion of the spectrum but rather ultraviolet. Thus what can be seen burning in the photographs cannot be hydrogen. However, black-and-white photographic film of the era had a different light sensitivity spectrum than the human eye, and was sensitive farther out into the infrared and ultraviolet regions than the human eye. And while hydrogen tends to burn invisibly, the materials around it, if combustible, would change the color of the fire.

The motion picture films show the fire spreading downward along the skin of the airship. While fires generally tend to burn upward, especially including hydrogen fires, the enormous radiant heat from the blaze would have quickly spread fire over the entire surface of the airship, thus apparently explaining the downward propagation of the flames. Falling, burning debris would also appear as downward streaks of fire.

Those skeptical of the incendiary paint hypothesis cite recent technical papers which claim that even if the airship had been coated with actual rocket fuel, it would have taken many hours to burn – not the 32 to 37 seconds that it actually took.[55]

Modern experiments that recreated the fabric and coating materials of the Hindenburg seem to discredit the incendiary fabric hypothesis.[56] They conclude that it would have taken about 40 hours[clarification needed] for the Hindenburg to burn if the fire had been driven by combustible fabric. Two additional scientific papers also strongly reject the fabric hypothesis.[55][clarification needed]

However these claims do not agree with the results the Mythbusters achieved on their Hindenburg special of their TV show and others[who?] feel the criticisms does not take into account the conditions that lead to firestorms, such as convection and ignition from radiant energy.

The most conclusive[clarification needed] proof against the fabric hypothesis is in the photographs of the actual accident as well as the many airships which were not doped with aluminum powder and still exploded violently. When a single gas cell explodes, it creates a shock wave and heat. The shock wave tends to rip nearby bags which then explode themselves. In the case of the Ahlhorn disaster on January 5, 1918, explosions of airships in one hangar caused the explosions of others in three adjoining hangars, wiping out all five Zeppelins at the base.[clarification needed]

The photos of the Hindenburg disaster clearly show that after the cells in the aft section of the airship exploded and the combustion products were vented out the top of the airship, the fabric on the rear section was still largely intact, and air pressure from the outside was acting upon it, caving the sides of the airship inward due to the reduction of pressure caused by the venting of combustion gases out the top.

The loss of lift at the rear caused the airship to nose up suddenly and the back to break in half (the airship was still in one piece), at that time the primary mode for the fire to spread was along the axial gangway which acted as a chimney, conducting fire which burst out the nose as the airship's tail touched the ground, and as seen in one of the most famous pictures of the disaster.

Current marker at the disaster site, shown with Hangar No. 1 in background

The actual site of the Hindenburg crash at Naval Air Station Lakehurst, renamed by the Naval Air Systems Command (NAVAIR) as Naval Air Engineering Station (NAES) Lakehurst (or "Navy Lakehurst" for short)[57] is marked with a chain-outlined pad and bronze plaque where the airship's gondola landed.[58] It was dedicated on May 6, 1987, the 50th anniversary of the disaster.[59]Hangar No.1, which still stands, is where the airship was to be housed after landing. It was designated a Registered National Historic Landmark in 1968.[60] Pre-registered tours are held through the Navy Lakehurst Historical Society.[61] Due to security concerns, no foreign nationals are permitted on the tours.[59]

^Per an annotated ship's diagram submitted to the U.S. Commerce Department's Board of Inquiry into the disaster, there were 12 men in the forward section of the ship at the time of the fire: Ludwig Felber (apprentice "elevatorman"); Alfred Bernhardt (helmsman); Erich Spehl (rigger); Ernst Huchel (senior elevatorman); Rudi Bialas (engine mechanic); Alfred Stöckle (engine mechanic); Fritz Flackus (cook's assistant); Richard Müller (cook's assistant); Ludwig Knorr (chief rigger); Josef Leibrecht (electrician); Kurt Bauer (elevatorman); and Alfred Grözinger (cook). Of these, only Leibrecht, Bauer and Grözinger survived the fire. Examination of the unedited Board of Inquiry testimony transcripts (stored at the National Archives), combined with examination of the Landing Stations chart on page 212 of Harold Dick and Douglas Robinson's book Graf Zeppelin and Hindenburg, indicates that the six off-watch men who were sent forward to trim the ship were Bialas, Stöckle, Flackus, Müller, Leibrecht and Grözinger. The other men were at their previously assigned landing stations. More recent research found that was not Bialas, but his colleague Walter Banholzer, who was sent forward along with the other five men.

^This is corroborated by the official testimonies and later recollections of several passenger survivors from the starboard passenger deck, including Nelson Morris, Leonhard Adelt and his wife Gertrud, Hans-Hugo Witt, Rolf von Heidenstam, and George Hirschfeld.

^Board of Inquiry testimony of Hans-Hugo Witt, a Luftwaffe military observer traveling as a passenger.

^Subsequent on-camera interviews with Späh and his letter to the Board of Inquiry corroborate this version of his escape. One or two more dramatic versions of his escape have appeared over the years, neither of which are supported by the newsreels of the crash, one of which shows a fairly close view of the portside passenger windows as passengers and stewards begin to drop through them.

^Although Joseph Späh survived the Hindenburg accident and fire, he was not as "lucky" when he appeared in the 1976 motion picture Marathon Man. Billed in the film under his longtime stage name, Ben Dova, Späh portrayed the brother of fugitive Nazi war criminal Dr. Christian Szell (Laurence Olivier). Späh's character is involved in an automobile accident early in the film in which his car collides with an oil delivery truck and he is burned to death in the ensuing fire.[21]

^Some of the 26 people listed as immediate victims may have actually died immediately after the disaster in the air station's infirmary, but being identified only after some time, along with the corpses of the victims who died in the fire.